1. Field of the Invention
The present invention relates to spread-spectrum communication in which a transmission signal is spread using a spreading code uniquely assigned to each terminal and is despread using an identical spreading code at the receiving end, thereby providing a plurality of communication channels using different spreading codes within the same frequency band.
2. Description of the Related Art
In recent years, portable terminals in mobile communication systems and personal computers and other data terminals have been spreading rapidly as cost reductions are made along with reductions in size. Against this backdrop, it is strongly demanded to provide communication services that enable large amounts and a large variety of information to be transmitted and received in outdoor situations and in mobile vehicles by linking these two systems. Research and work for commercialization of techniques required to implement such communication services have been proceeding, with effort directed toward utilizing the techniques employed in the currently commercialized analog and digital mobile communication systems as far as possible.
Of these techniques, selective diversity, equal-ratio combining, maximal-ratio combining, and other space diversity methods, for example, have already been used widely in practical applications as techniques for compensating for the transmission characteristics that greatly fluctuate with the movement of mobile units and because of reflections from terrestrial surfaces and terrestrial objects intervening in a radio transmission path and the attenuation resulting from such obstacles.
In space diversity methods, to obtain a sufficient effect of space diversity the correlation between received waves arriving at a plurality of branches (antennas) must be minimized, which requires that minimum required spacing be provided between these branches. This has served as a limiting factor in reducing hardware size at the receiving end, and often has precluded its practical application in portable-type terminals among others.
One approach to overcoming such constraints may be employing a system in which transmission information is first converted into parallel sequences and then combined for transmission after modulation with Hadamard sequences, as disclosed, for example, in Japanese Patent Unexamined Publication No. 7-274249. In this system, however, since the transmission signal is a multi-valued signal, high linearity is required of amplifiers and other hardware constituting the transmitter and receiver, which has placed limitations on the selection of circuits, mounting, etc. and often has precluded its practical application.
As techniques for achieving such compensation, spread-spectrum communication systems are known in which transmission information having a bit rate of several tens of kbps is spread over a bandwidth of several MHz to several tens of MHz and transmitted through a radio link, and the transmitted information is despread at the receiving end, thus making it possible to avoid the effects of delayed waves arriving with delays of several tens of nanoseconds to several microseconds.
However, in the prior known spread-spectrum communication systems, in applications that require achieving high-quality transmission by avoiding the effects of received waves having a small amount of delay such as Rayleigh fading, information to be transmitted has to be spread over a wider bandwidth (which means a greater bandwidth expansion factor); therefore, practical application has not been possible in many cases due to such limiting factors as a significant increase in power consumption and the limited number of assignable radio frequencies.